Quantum information techniques constitute a technology or a field of the technology that utilizes a quantum mechanical effect directly to achieve information processing performance unachievable so far. Quantum entanglement is a most important resource in the quantum information techniques. Utilization of the quantum entanglement permits actualizing absolutely safe communications and computation processing at a speed incommensurably higher than heretofore.
A quantum entangled state is a state that physical systems at a plurality of spatially separated locations are mutually correlated, thus the state that such a plurality of physical systems cannot be treated isolated. If physical systems at two distant locations have a quantum entangled state in common, then measurements conducted at the two locations cause in their results a correlation which cannot be explained in the classical theory.
The term “quantum entanglement” is used in general to refer to a quantum entangled state itself, or a physical phenomenon which the entangled state exhibits and which is brought about peculiar in the quantum theory, or to state the concept that the quantum theory involves an inseparable characteristic. The quantum entanglement is used herein, however, as the term to indicate a quantum entangled state.
Quantum information processing adopts mainly two approaches, one of which uses a discrete physical quantity and the other of which uses a continuous physical quantity (see, e.g., Non-Patent Reference 1). In the case of light, use is generally made of the quadrature amplitude of an electric field as such a physical quantity taking a continuous physical value. The quantum entanglement for continuous physical quantities is termed a continuous variable quantum entanglement.
Mention is made of conventional methods of generating a continuous variable quantum entanglement. The method used most initially uses a non-degenerate parametric amplifier (see, e.g., Patent Reference 1). Patent Reference 1 introduced an experiment in which potassium titanate phosphate (KTP) was used as a nonlinear medium and phase matching of type II was effected to generate a signal and an idler light beams which are in a mutually orthogonal polarized state. The term “non-degenerate” refers to difference in the polarized state. Such signal and idler light beams as generated by parametric amplification using phase matching of type II are quantum correlated and thus capable of generating a continuous variable quantum entanglement.
In a conventional method of using the phase matching of type II, however, a difference in index of reflection of the nonlinear medium to signal and idler light beams made it technically difficult to bring the light resonators into simultaneous resonance with these two light beams. Further, the phase matching of type II in which beams tended in general to work off caused the quantum entanglement to deteriorate in quality.
In the method next performed, two squeezed light beams are generated and combined at a beam splitter with a transmissivity and a reflectance both of 50% to generate quantum entanglement. Then, the two squeezed beams need to be precisely controlled so as to have their relative phase difference of π/2.
For example, refer to Non-Patent Reference 2 in which a parametric amplifier placed in a ring resonator to effect phase matching of type 1 is used to generate squeezed beams which are traveling clockwise and anticlockwise along a ring and which are combined at a beam splitter laid outside of the ring to generate quantum entanglement. This method has the problem that after leaving the ring resonator and then to be combined at the beam splitter, the two squeezed beams that follow the different paths make it difficult to maintain the relative optical path length between these two paths stably.
Patent Reference 1: H. J. Kimble et al., U.S. Pat. No. 5,339,182, Aug. 16, 1994
Non-Patent Reference 1: S. L. Braunstein and P. van Loock, Rev. Mod. Phys. Vol. 77, p. 513, 2005
Non-Patent Reference 2: T. C. Zhang, et al., Phys. Rev. A. Vol. 67, p. 033802, 2003
Non-Patent Reference 3: Yujiro Eto, et al., Optics Letters, Vol. 32, pp. 1698-1700, 2007
Non-Patent Reference 4: L. M. Duan, et al., Physical Review Letters, Vol. 84, p. 2722, 2000